What caused Hunga Tonga-Hunga Ha'apai to erupt? We ask an expert

Abongile Nzelenzele speaks to Prof Raymond Cas, professor of volcanology about the Tonga underwater volcano.

- The four-foot tsunami hit the South Pacific nation of Tonga on Saturday

- The eruption of the Hunga Tonga-Hunga Haʻapai volcano was heard across the region and as far away as the US

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It's five days since Hunga Tonga-Hunga Ha'apai exploded, covering Tonga in a film of ash and triggering a Pacific-wide tsunami.

Prof Raymond Cas, professor of volcanology at Monash University explains what caused the underwater volcano to erupt.

He says previous eruptions have been taking place over the past twenty years on a smaller scale.

Hot magma is coming up in the vent of the volcano from the Earth's interior, which is at a temperature of 1 000 degrees, explosively super-heating the shallow seawater it is erupting into.

Prof Raymond Cas, Professor of volcanology - Monash University

It's that super-heating of the seawater that instantly vaporises, that drove those small eruptions.

Prof Raymond Cas, Professor of volcanology - Monash University

Cas says last Saturday's eruption was unlike previous eruptions:

My assessment would be, that suddenly, a very large volume of magma...rose in the vent, rose very quickly and that magma contained a very high dissolved volcanic gas component.

Prof Raymond Cas, Professor of volcanology - Monash University

When that erupts at shallow seawater depths, those gases can expand extremely intensely and that then produced this column of gas and ash that rose 20 kilometres into the atmosphere.

Prof Raymond Cas, Professor of volcanology - Monash University

Hunga Tonga-Hunga Ha'apai forms part of a chain of marine volcanos called the Tongan Arc.

In that chain of volcanos, there are many that are totally underwater, Hunga Tonga-Hunga Ha'apai over its history has grown from a water depth on the floor of the pacific at 2km.

Prof Raymond Cas, Professor of volcanology - Monash University

Cas says these types of marine volcanos are not limited to the pacific region but are also found in places like the Mediterranean sea.

RELATED: [VIDEOS] Undersea eruption unleashes tsunami on Tonga, Aus evacuates Bondi Beach

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